While the most common explanation of many mens' tendency to fetishize women having sex with each other (one woman is sexy, two women is even sexier) probably plays a major role, there are some more psychologically complex suggestions.
* The conflicted attitudes some straight men have to male porn stars in male/female porn. Some men are resistant to porn with attractive men in it, either because the handsome men make them feel inferior or because they are worried [[StupidSexyFlanders that they might end up finding the men hot]]. This leads some porn producers to deliberately cast men who are much less attractive than the women, which on the other hand ends up repelling some male viewers on aesthetic grounds.
* The hope that female/female porn may somehow reveal awesome secret methods of giving women pleasure that men don't inherently know. This is probably a false hope given the lack of realism in porn. For example, tribadism (look it up) is often reported as an extremely popular activity in sex surveys in real lesbian media, but is very rarely found in male-audience female/female porn because of its inherent lack of spectator appeal.
* While this is hopefully dying out (except among some Mens' Rights Activist circles, where it is still common), examination of twentieth-century "shocking" fiction will reveal a common fear among men that women don't actually enjoy sex, but only do it in exchange for material goods, emotional support, or conception of children. Hence, if two women are having sex together, the obvious implication is that at least one of them is doing it because she genuinely wants to.